(1) Field of the Invention
The present invention relates to an optical semiconductor device and an optical pickup device, and more particularly, to an optical semiconductor device which converts light into an electric signal and outputs the electric signal.
(2) Description of the Related Art
In recent years, the increase in the size of circuits is progressing with the increase in integration, function and speed of semiconductor integrated circuit devices. Accordingly, wirings become longer, and frequency characteristic deterioration due to the parasitic capacitance of the wirings is becoming a problem.
An optical pickup device includes a laser generator which irradiates an optical disk medium with a laser light used for each of a CD (Compact Disk) and a DVD (Digital Versatile Disc), and an optical semiconductor device which receives and amplifies the reflected light. The optical semiconductor device includes plural light-receiving elements which convert the received light into a photo-electric current, and an amplifier with several channels, which converts the photo-electric current received from the light-receiving elements into voltage.
As a light source, an infrared laser is used for the CD and a red laser is used for the DVD. Recently, a dual wavelength laser generator which monolithically generates the infrared laser and the red laser has become widespread. Positions for emitting the respective lasers having different wavelengths are provided at a predetermined interval in the dual wavelength laser generator, so that there exist two types of optical axis. Thus, the optical semiconductor device which receives the reflected light from the optical disk medium must have specific light-receiving elements and the amplifier circuits to accept the respective wavelengths. Consequently, the number of channels in the amplifier provided on one semiconductor substrate increases.
To cope with the deterioration of the frequency characteristic in the optical semiconductor device, a method of forming a shield wire for wiring which connects the light-receiving element and an input of the amplifier is well known (refer to Patent Document 1, for example).
The shield wire in the optical semiconductor device according to Patent Document 1 is described below. FIG. 1 is a diagram showing a configuration of the conventional optical semiconductor device according to Patent Document 1. The optical semiconductor device shown in FIG. 1 includes a light-receiving element 51 which is placed in a metal case 54, a semiconductor integrated circuit 63 which is formed on a print board 64, and inner leads 56 and 57. The light-receiving element 51 converts a light signal into a photo-electric current. The semiconductor integrated circuit 63 converts the photo-electric current which is received from the light-receiving element 51 into a voltage. The semiconductor integrated circuit 63 includes an amplifier 52 and a voltage follower 53. The amplifier 52 converts the photo-electric current which is received from the light-receiving element 51 into a voltage. The voltage follower 53 buffers a potential of a node 62 (an input wire 60 of the amplifier 52) and then outputs the potential to the node 61. The inner lead 57 connects a connection wire 55 of the light-receiving element 51 and an input wire 60 of the amplifier 52 through a terminal 58. The inner lead 56 is connected to the node 61 through a terminal 59 and is placed on both sides and a downside of the inner lead 57 (the downside is not shown). The inner lead 56 is positioned to eliminate a parasitic capacitance, which is generated in the inner lead 57, with the print board and to generate a parasitic capacitance with the inner lead 56. A potential of the inner lead 56 is equal to that of the inner lead 57, so that even when a parasitic capacitance is generated, the inner leads 56 and 57 operate in phase with an alternating current signal, and thus the deterioration of the frequency characteristic which is caused by the parasitic capacitance for the inner lead 57 is prevented.
Patent Document 1: Japanese Patent Publication No. 3103175.